1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to manufacturing a laterally graded porous silicon optical filter, particularly, to a method of manufacturing a porous silicon film having a lateral etch-depth gradient through diffusion-limited etching (DLE), and more particularly, to a method of manufacturing an optical filter, in which etching through diffusion control using a porous silicon etch bath is carried out with the use of a mask having a tapered window opening, so that the resonance frequency changes in a taper axis direction.
2. Description of the Related Art
Typically, porous silicon (PSi) has continuously received attention in the field of photonics since it was first discovered. This attention is being paid to PSi because of the light emitting properties thereof, but properties and characteristics essential for manufacturing passive photonic devices having high functionality, such as optical mirrors, Fabry-Perot filters, and waveguides, should be considered.
The application of PSi to passive devices advantageously results in in-situ control of optical parameters thereof. In the course of forming PSi through anodic etching, a refractive index n may be adjusted depending on the changes in current density, whereas a physical thickness d is directly adjusted by an etching duration time. The optical thickness (OT) of a thin film, which is an important parameter for designing an optical structure, may be easily adjusted during the etching process. Further, it appears that the gradual change of OT in a spatial region is useful in the formation of passive photonic devices using PSi.
The vertical control of OT may be easily achieved, depending on the changes in etch current and/or duration time, but lateral gradation is not easy. In order to create the lateral gradation in the PSi layers, various methods have been developed to date, including the use of an in-plane electric field.
In addition, circular anodic metal is used to form a gradient index lens (GRIN lens).
The common point of the above methods is that the lateral gradation of the film properties is determined by a hole current distribution in a wafer.